Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare a non-read-only, non-destructive, non-idempotent write, and the description adds real substance beyond them: library-origin sessions update in place with the prior version retained in history, other sessions create new sprites, and repeat saves update rather than duplicate. It omits permissions/auth requirements and what the call returns.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three dense sentences, front-loaded with the action and scope, then the branching rules. No filler, though the parenthetical listing of session sources and defaults makes it slightly heavy for a four-parameter tool.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Covers the mutation semantics, overwrite/history behavior, and defaulting rules needed to call it correctly. With no output schema, the description does not say what comes back (e.g. the sprite id) or how the saved sprite is named, which leaves a small gap.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so all four parameters are already documented. The description adds only the behavioral interaction (as_new overrides library-origin update; project_id defaults to the most recently used project), which largely restates the schema's default text.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

Specific verb+resource: save the sprite from a SpriteForge edit session into the user's library. It names the session origins (open_editor, open_sprite_in_editor, pasted edit link), which distinguishes it from save_art / upload_sprite without needing those schemas.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Clearly states when this applies (any session, with the library-origin case spelled out) and how as_new overrides that. It does not name sibling alternatives such as save_art or upload_sprite, so the agent must infer which tool handles non-session artwork.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.